Ozone troposphere/photochemical

As discussed in other chapters of this book and summarized in Chapter 16, the formation of tropospheric ozone from photochemical reactions of volatile organic compounds (VOC) and oxides of nitrogen (NC/) involves many reactions. Concentrations are therefore quite variable geographically, temporally, and altitudinally. Additional complications come from the fact that there are episodic injections of stratospheric 03 into the troposphere as well as a number of sinks for its removal. Because 03 decomposes thermally, particularly on surfaces, it is not preserved in ice cores. All of these factors make the development of a global climatology for 03 in a manner similar to that for N20 and CH4, for example, much more difficult. In addition, the complexity of the chemistry leading to O, formation from VOC and NOx is such that model-predicted ozone concentrations can vary from model to model (e.g., see Olson et al., 1997). 

In the literature a wide range of estimates regarding the influx of ozone from the stratosphere into the troposphere is presented, derived by many different methods. Some studies report an annual flux between 200-870 Tg 03 yr 1 globally, whereas other studies report a flux between 500 and 1000 Tg 03 yr 1 for the NH only. With our model we estimate for the NH a net downward flux from the stratosphere of 580 Tg 03 yr1, which is partly balanced by an upward flux of ozone from photochemical production in the troposphere of 210 Tg 03 yr1, yielding a net downward cross-tropopause ozone transport of 370 Tg 03 yr 1. Globally, these values are 950, 370 and 580 Tg 03 yr 1, respectively. (Note that the troposphere-to-stratosphere ozone flux in the... 

The Photochemical Activity and solar Ultraviolet Radiation (PAUR I) and Photochemical Activity and solar Ultraviolet Radiation Modulation Factors (PAUR II) projects had the aim of studying various aspects of ultraviolet radiation and photochemistry interrelationships. PAUR I aimed at studying the interrelationships between total ozone, UV-B radiation, aerosol load, air pollutants, photodissociation rates of N02 and 03 and tropospheric ozone. PAUR II has the aim of studying the interactions between UV-B, total ozone, tropospheric ozone and photochemical activity in the presence of alternating maritime and Saharan aerosols. The present paper presents the main concepts underlying the two projects, the approach followed and a brief overview of some of the results obtained so far. Further, the main results of PAUR I that are relevant to tropospheric ozone chemistry over the Eastern Mediterranean are presented. 

The photodissociation of N02 is the only definitely established process for the formation of ozone in the troposphere and must be held responsible also for the generation of ozone in photochemical smog. The reaction of alkylperoxy radicals with oxygen, ROO + 02— R0 + 03 has occasionally been invoked—for example, by Cadle and Allen (1970)—but according to... 

The tacit assumption made in the preceding discussion that ozone is photochemically stable in the troposphere is basically incorrect, and we must finally consider this aspect. It is true that the photodissociation of ozone as far as it leads to 0(3P) atoms causes no losses, because their subsequent attachment to molecular oxygen regenerates ozone. In Section 4.2 it was shown, however, that a part of the O( D) atoms produced in the... 

Methane in the troposphere contributes to the photochanical production of carbon monoxide and ozone. The photochemical oxidation of methane is a major source of water vapor in the stratosphere. 

The first point is due to the incomplete treatment of the pure oxygen theory, ignoring trace constituents other than O, O2 and O3, which will be described in detail below. The second point is due to the transport of ozone within the stratosphere and into the troposphere. Photochemical lifetime of stratospheric O3 is the order of 10 min at the altitude of 45 km and the diurnal cycle is observed. On the... 

Ait Quality Criteria for Ozone and Related Photochemical Oxidants. Volume 1.3. Tropospheric Ozone and Its Precursors. Research Triangle Park, NC F.PA (1996). http //www.epa.gov.ncc.i/ ozone.htm. 

About 51 percent of solar energy incident at the top of the atmosphere reaches Earth s surface. Energetic solar ultraviolet radiation affects the chemistry of the atmosphere, especially the stratosphere where, through a series of photochemical reactions, it is responsible for the creation of ozone (O,). Ozone in the stratosphere absorbs most of the short-wave solar ultraviolet (UV) radiation, and some long-wave infrared radiation. Water vapor and carbon dioxide in the troposphere also absorb infrared radiation. 

Certainly, photochemical air pollution is not merely a local problem. Indeed, spread of anthropogenic smog plumes away from urban centers results in regional scale oxidant problems, such as found in the NE United States and many southern States. Ozone production has also been connected with biomass burning in the tropics (79,80,81). Transport of large-scale tropospheric ozone plumes over large distances has been documented from satellite measurements of total atmospheric ozone (82,83,84), originally taken to study stratospheric ozone depletion. 

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